Method of increasing the oxidation stability of biodiesel

ABSTRACT

The invention relates to a method of increasing the oxidation stability of biodiesel, which comprises adding a primary antioxidant having a melting point of less than or equal to 40° C. to the biodiesel to be stabilized in an amount of from 10 to 20 000 ppm (w/w), where the primary antioxidant comprises at least one compound having the structure  
                 
where: 
         R 1 , R 2 =hydrogen, a linear alkyl group having from 1 to 20 carbon atoms or  
                 
where * is a carbon atom of the aromatic ring system,    R 3 , R 5 =hydrogen, a linear alkyl group having from 1 to 20 carbon atoms,    R 4 =hydrogen, a linear alkyl group having from 1 to 40 carbon atoms, 
 
where both the substituents of the type R 1  and R 2  and those of the type R 3  and R 5  are in each case identical or different.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method of increasing the oxidation stabilityof biodiesel. The invention also includes the oxidation-stabilizedproduced thereby.

2. Description of the Background

An alternative to conventional diesel fuel which is being used to anincreasing extent today is biodiesel, which comprises monoalkyl estersof vegetable oils, animal fats and also used cooking fats. Biodiesel isobtained by transesterification of oils, for example rapeseed oil,soybean oil or sunflower oil and also used cooking oils, with an alcoholin the presence of a catalyst.

Since the importance of biodiesel as an alternative diesel fuel forpassenger cars has been continually increasing in importance in recenttimes, the production of biodiesel has also increased to a correspondingextent in recent years. Biodiesel has a high content of unsaturatedfatty acid esters which can easily be oxidized by atmospheric oxygen.The products formed (including acids, resins) can lead to corrosion andblockages in injection pumps and/or fuel lines. The increasing use ofthe alternative biodiesel as automobile fuel has led to a need foroxidation-stabilized biodiesel. According to the prior art, preferenceis given to adding 2,6-di-tert-butyl-4-methylphenol (BHT) as primaryantioxidant to the biodiesel in order to meet the oxidation stabilityrequirements of the standard DIN EN 14214.

The use of 2,6-di-tert-butyl-4-methylphenol as antioxidant is describedin the European patent EP 0 189 049. Here, the use of2,6-di-tert-butyl-4-methylphenol in amounts of from 10 to 100 ppm asexclusive stabilizer in methyl esters of palm kernel oil having from 12to 18 carbon atoms in the fatty acid is described.

DE 102 52 714 and WO 2004/044104, too, describe a method of increasingthe oxidation stability of biodiesel by addition ofdi-2,6-tert-butyl-4-hydroxytoluene. A liquid stock solution containingfrom 15 to 60% by weight of monoalkylhydroxytoluene ordialkyl-hydroxytoluene dissolved in biodiesel is added to the biodieselto be stabilized to give a concentration of 0.005 to 2% by weight ofmonoalkylhydroxytoluene or dialkylhydroxy-toluene, based on the totalsolution in biodiesel.

DE 102 52 715 describes a method of increasing the storage stability ofbiodiesel, in which a liquid stock solution containing from 15 to 60% byweight of 2,4-di-tert-butyl-hydroxytoluene dissolved in biodiesel isadded to the biodiesel to be stabilized to give a concentration of from0.005 to 2% by weight of 2,4-di-tert-butylhydroxytoluene, based on thetotal solution in biodiesel.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improved methodof increasing the oxidation stability of biodiesel. In particular, it isan object of the invention to improve the processability of the primaryantioxidant in biodiesel and thus improve the economics or the use ofprimary antioxidants in biodiesel.

It has surprisingly been found that the addition of a liquid primaryantioxidant based on substituted alkylphenols having the structure I tobiodiesel makes it possible to improve the handleability of the primaryantioxidants in biodiesel. The primary antioxidant used in the method ofthe invention has a low melting point, i.e. a melting point of less thanor equal to 40° C. Owing to its low melting point, this primaryantioxidant can, in the method of the invention, be mixed as liquiddirectly into the biodiesel without this primary antioxidant firstlyhaving to be dissolved in a solvent, for example alcohol, or inbiodiesel. Thus, the preparation of a masterbatch of this primaryantioxidant in biodiesel can be dispensed with. The masterbatches aregenerally prepared using the biodiesel for which the masterbatch isready to be used. In the case of large distances between biodiesel andantioxidant manufacturers in particular, this leads to high costs forstorage and transport in the case of primary antioxidants according tothe prior art. The primary antioxidant used in the method of theinvention thus leads to low production costs for theoxidation-stabilized biodiesel, since, firstly, the preparation ofsolutions or masterbatches comprising the active ingredient primaryantioxidant can be dispensed with and, secondly, the present methodshows that a product mixture from the manufacturing process ofsubstituted alkylphenols can be used without complicated separation andpurification steps. A further advantage of the method of the inventionis that the production of the oxidation-stabilized biodiesel can becarried out at low temperatures and within a relatively short time. Themethod of the invention thus makes it possible to improve the economicsof the production of oxidation-stabilized biodiesel. The active contentof the liquid primary antioxidant used according to the presentinvention is 100% compared to 20% of the liquid primary antioxidantsaccording to the prior art, since the liquid generally contains onlyabout 20% by weight of the active ingredient antioxidant. The use of aliquid primary antioxidant according to the method of the inventionlikewise enables transport and storage costs to be saved as a result ofthe high active content of primary antioxidant per liter. The use of aliquid primary antioxidant according to the method of the invention alsoleads to reduced crystallization of the primary antioxidant in thebiodiesel or in plant components at low temperatures.

The present invention provides a method of increasing the oxidationstability of biodiesel, which comprises adding a primary antioxidanthaving a melting point of less than or equal to 40° C. to the biodieselto be stabilized in an amount of from 10 to 20 000 ppm (w/w), where theprimary antioxidant comprises at least one compound having the structure

where:

-   -   R₁, R₂=hydrogen, a linear alkyl group having from 1 to 20 carbon        atoms or        where * is a carbon atom of the aromatic ring system,    -   R₃, R₅=hydrogen, a linear alkyl group having from 1 to 20 carbon        atoms,    -   R₄=hydrogen, a linear alkyl group having from 1 to 40 carbon        atoms, where both the substituents of the type R₁ and R₂ and        those of the type R₃ and R₅ are in each case identical or        different.

The invention further provides for the use of a primary antioxidantwhich has a melting point of less than or equal to 40° C. and comprisesat least one compound having the structure

where:

-   -   R₁, R₂=hydrogen, a linear alkyl group having from 1 to 20 carbon        atoms or        where * is a carbon atom of the aromatic ring system,    -   R₃, R₅=hydrogen, a linear alkyl group having from 1 to 20 carbon        atoms,    -   R₄=hydrogen, a linear alkyl group having from 1 to 40 carbon        atoms, where both the substituents of the type R₁ and R₂ and        those of the type R₃ and R₅ are in each case identical or        different, for increasing the oxidation stability of biodiesel.

The invention likewise provides an oxidation-stabilized biodieselcomprising from 10 to 20 000 ppm (w/w) of a primary antioxidant whichhas a melting point of less than or equal to 40° C. and comprises atleast one compound having the structure

where:

-   -   R₁, R₂=hydrogen, a linear alkyl group having from 1 to 20 carbon        atoms or        where * is a carbon atom of the aromatic ring system,    -   R₃, R₅=hydrogen, a linear alkyl group having from 1 to 20 carbon        atoms,    -   R₄=hydrogen, a linear alkyl group having from 1 to 40 carbon        atoms, where both the substituents of the type R₁ and R₂ and        those of the type R₃ and R₅ are in each case identical or        different.

The inventive method of increasing the oxidation stability of biodieselcomprises adding a primary antioxidant having a melting point or lessthan or equal to 40° C. to the biodiesel to be stabilized in an amountof from 10 to 20 000 ppm (w/w), where the primary antioxidant comprisesat least one compound having the structure

where:

-   -   R₁, R₂=hydrogen, a linear alkyl group having from 1 to 20 carbon        atoms or        where * is a carbon atom of the aromatic ring system,    -   R₃, R₅=hydrogen, a linear alkyl group having from 1 to 20 carbon        atoms,    -   R₄=hydrogen, a linear alkyl group having from 1 to 40 carbon        atoms, where both the substituents of the type R₁ and R₂ and        those of the type R₃ and R₅ are in each case identical or        different.

DETAILED DESCRIPTION OF THE INVENTION

For the purposes of the present invention, primary antioxidants arecompounds or mixtures of compounds which inhibit or prevent undesirabledegradation reactions in the biodiesel caused by oxygen. The mode ofaction of these primary antioxidants in the biodiesel is described inthe following reaction scheme, where R and R′ are each an organicradical and AOH is a primary antioxidant used in the method of theinvention.

-   -   1. Chain initiation        R—H→R•+H•        R—H+O₂→R•+HO₂•    -   2. Chain propagation        R•+O₂→RO₂•        RO₂•+R′—H→ROOH+R′•    -   3. Chain termination        RO₂•+AOH→ROOH+AO•        AO•+R•→AOR

Apart from the abovementioned reactions, it is also possible forreactions on the double bonds of the alkyl esters of fatty acids, whichcan likewise be initiated by oxygen, to occur. Here, the carbon-hydrogenbond which is located in the allyl position relative to the double bondis preferentially attacked by the oxygen:

The primary antioxidant used in the method of the invention comprisesneither organic solvents, for example alcohols, nor biodiesel, as is thecase, for example, when a masterbatch is employed. The primaryantioxidant used therefore preferably comprises exclusively alkylphenolsand/or substituted alkylphenols. In particular, a mixture comprisingvarious substituted and/or unsubstituted alkylphenols, preferably amixture comprising from 0.1 to 99.9% by weight, preferably from 50 to99% by weight and particularly preferably from 55 to 90% by weight, ofone or more compounds having the structure I, is used as primaryantioxidant in the method of the invention.

Particular preference is given to using a mixture comprising exclusivelycompounds having the structure I as primary antioxidant in the method ofthe invention. This mixture has the advantage that it comprisesexclusively compounds having the structure I which all have anoxidation-stabilizing action on biodiesel.

In the method of the invention, preference is given to using a primaryantioxidant comprising at least one compound having the structure II

where:

-   -   R₁, R₂=hydrogen, a linear alkyl group having from 1 to 4 carbon        atoms or        where * is a carbon atom of the aromatic ring system,    -   R₃, R₅=hydrogen, a linear alkyl group having from 1 to 4 carbon        atoms,    -   R₄=hydrogen, a linear alkyl group having from 1 to 4 carbon        atoms, where both the substituents of the type R₁ and R₂ and        those of the type R₃ and R₅ are in each case identical or        different.

Preference is given to adding a primary antioxidant comprising at leastone compound having the structure III

to the biodiesel in the method of the invention.Particular preference is given to adding a primary antioxidantcomprising at least one compound having the structure IV

to the biodiesel in the method of the invention.

Furthermore, the primary antioxidant used in the method of the inventioncan comprise exclusively compounds selected from among

-   -   tri-tert-butylphenols, in particular 2,4,6-tri-tert-butylphenol,    -   di-tert-butylphenols, in particular 2,4-di-tert-butylphenol,        2,5-di-tert-butylphenol, 2,6-di-tert-butylphenol,    -   di-tert-butylmethylphenols, in particular        2,5-di-tert-butyl-4-methylphenol,        2,6-di-tert-butyl-4-methylphenol,        4,6-di-tert-butyl-2-methylphenol, di-tert-butyl-3-methylphenol,    -   tert-butylmethylphenols, in particular        2-tert-butyl-4-methylphenol, 6-tert-butyl-2-methylphenol,        4-tert-butyl-2-methylphenol, 6-tert-butyl-3-methylphenol,    -   tert-butyldimethylphenols, in particular        4-tert-butyl-2,6-dimethylphenol,        6-tert-butyl-2,4-dimethylphenol, tert-butyl-2,5-dimethylphenol,    -   tert-butylphenols, in particular 2-tert-butylphenol,        4-tert-butylphenol,    -   di-sec-butylphenols,    -   sec-butylphenols, in particular 2-sec-butylphenol,        2-sec-butyl-4-tert-butylphenol,        4-sec-butyl-2,6-di-tert-butylphenol,    -   tert-amylphenols, in particular 2-tert-amylphenol,    -   di-tert-amylphenols, in particular 2,4-di-tert-amylphenol,    -   2-isopropylphenol, 4-octylphenol, 4-nonylphenol,        2,6-di-tert-butyl-4-nonylphenol, 4-dodecylphenol and/or        octadecylphenol,        and mixtures of these compounds, with this primary antioxidant        having a melting point of less than or equal to 40° C.

Pure substances or mixtures which are or comprise compounds having thestructures I to IV and have a melting point of greater than 40° C. areexcluded from the present invention.

The primary antioxidant used in the method of the invention has amelting point of ≦40° C., preferably ≦39° C. and particularly preferably≦38° C.

Secondary antioxidants which can be used in the method of the inventionare alkylthiomethylphenols, preferably selected from among

-   -   2,4-di((octylthio)methyl)-6-tert-butylphenol,    -   2,4-di((octylthio)methyl)-6-methylphenol,    -   2,4-di((octylthio)methyl)-6-ethylphenol and    -   2,6-di((dodecylthio)methyl)-4-nonylphenol,        hydroxylated diphenyl thioethers, preferably selected from among    -   2,2′-thiobis[6-tert-butyl-4-methylphenol],    -   2,2′-thiobis[4-octylphenol],    -   4,4′-thiobis[6-tert-butyl-3-methylphenol],    -   4,4′-thiobis[6-tert-butyl-2-methylphenol],    -   4,4′-thiobis[3,6-di-sec-amylphenol] and    -   4,4′-bis[2,6-dimethyl-4-hydroxyphenyl]disulfide,        phosphites or phosphonites, preferably selected from among    -   triphenyl phosphite,    -   diphenyl alkyl phosphites,    -   phenyl dialkyl phosphites,    -   tris[nonylphenyl] phosphite,    -   trilauryl phosphite,    -   trioctadecyl phosphite,    -   distearyl pentaerythrityl diphosphite,    -   tris[2,4-di-tert-butylphenyl] phosphite,    -   diisodecyl pentaerythrityl diphosphite,    -   bis[2,4-di-tert-butylphenyl] pentaerythrityl diphosphite,    -   bis[2,6-di-tert-butyl-4-methylphenyl] pentaerythrityl        diphosphite,    -   bis[isodecyloxy] pentaerythrityl diphosphite,    -   bis[2,4-di-tert-butyl-6-methylphenyl] pentaerythrityl        diphosphite,    -   bis[2,4,6-tri-tert-butylphenyl] pentaerythrityl diphosphite,    -   tristearyl sorbitol triphosphite,    -   tetrakis[2,4-di-tert-butylphenyl] 4,4′-biphenylenediphosphonite,    -   6-isooctyloxy-2,4,8,10-tetra-tert-butyl-12H-dibenzo[d,g]-1,3,2-dioxaphosphocine,    -   6-fluoro-2,4,8,10-tetra-tert-butyl-12-methyldibenzo[d,g]-1,3,2-dioxaphosphocine,    -   bis[2,4-di-tert-butyl-6-methylphenyl] methyl phosphite and    -   bis[2,4-di-tert-butyl-6-methylphenyl] ethyl phosphite,        or peroxide-destroying compounds, preferably selected from among    -   esters of β-thiodipropionic acid, preferably the lauryl,        stearyl, myristyl or tridecyl ester,    -   mercaptobenzimidazole,    -   the zinc salt of 2-mercaptobenzimidazole,    -   zinc dibutyldithiocarbamate,    -   dioctadecyl disulfide and    -   pentaerythrityl tetrakis[β-dodecylmercapto]propionate,        or mixtures of these compounds.

For the purposes of the present invention, the term biodieselencompasses all the saturated and/or unsaturated alkyl esters of fattyacids, in particular methyl or ethyl esters of fatty acids, which can beused as energy carriers. For the purposes of the present invention,energy carriers include both fuels as sources of heat, for exampleheating material, and fuels for powering vehicles, for exampleautomobiles, goods vehicles, ships or aircraft. The biodiesel to whichthe method of the invention is applied is preferably a biodiesel whichis usually marketed under the name biodiesel for use as automobile fuel.In particular, the biodiesel to which the method of the invention isapplied comprises C₁₂-C₂₄ fatty acid alkyl esters, preferably C₁₂-C₂₄fatty acid methyl esters or C₁₂-C₂₄ fatty acid ethyl esters, which canbe present in pure form or as a mixture. In addition, the biodiesel towhich the method of the invention is applied can further comprise allcustomary additives such as secondary antioxidants, antifoams,low-temperature flow improvers. The method of the invention ispreferably applied to biodiesel produced from vegetable and/or animaloils by a process of transesterification with an alcohol, preferablymethanol or ethanol, particularly preferably methanol. The method of theinvention is more preferably applied to biodiesel comprisingtransesterification products of rapeseed oil, soybean oil, sunfloweroil, palm kernel oil, coconut oil, jatropha oil, cotton seed oil, peanutoil, maize oil and/or used cooking oils. However, particular preferenceis given to using biodiesel which is obtained from rapeseed oil,sunflower oil or soybean oil by means of the abovementionedtransesterification. The method of the invention can also be applied tomixtures of the transesterification products of various vegetable and/oranimal oils.

In a particular embodiment of the method of the invention, mixtures ofsaturated and/or unsaturated fatty acid alkyl esters with liquid energycarriers, for example mineral diesel fuel or heating oil, can be used asbiodiesel. Particular preference is given to using a mixture of mineraldiesel fuel and from 0.1 to 99.9% by volume, in particular from 2 to 10%by volume and preferably from 3 to 5% by volume, of saturated and/orunsaturated fatty acid alkyl esters. In a subsequent step of the methodof the invention, the oxidation-stabilized biodiesel can be added in anamount of from 0.1 to 99.9% by volume, in particular from 1 to 20% byvolume, preferably from 2 to 10% by volume and more preferably from 3 to5% by volume, to a liquid energy carrier, in particular mineral dieselfuel or heating oil.

In the method of the invention, the primary antioxidant is preferablyadded directly, in particular in an amount of from 10 to 20 000 ppm(w/w), preferably from 50 to 12 000 ppm (w/w) and more preferably from100 to 8000 ppm (w/w). For the purposes of the present invention, directaddition means that no preparation of a solution or a masterbatch of theprimary antioxidant has been carried out in a preceding step. In thisstep, secondary antioxidants can also be added in an amount of from 10to 20 000 ppm (w/w), preferably from 50 to 12 000 ppm (w/w) andpreferably from 100 to 8000 ppm (w/w), to the biodiesel.

The primary antioxidant is preferably dissolved in the biodiesel withstirring at a temperature of from 18° C. to 60° C., more preferably from20° C. to 40° C.

The invention further provides for the use of a primary antioxidantwhich has a melting point of less than or equal to 40° C. and comprisesat least one compound having the structure I for increasing theoxidation stability of biodiesel.

Particular preference is given to using a mixture comprising exclusivelycompounds having the structure I as primary antioxidant. This mixturehas the advantage that it comprises exclusively compounds having thestructure I which all have an oxidation-stabilizing action on biodiesel.Preference is given to using a primary antioxidant comprising at leastone compound having the structure II. However, greater preference isgiven to using a primary antioxidant comprising at least one compoundhaving the structure III. Particular preference is given to using aprimary antioxidant comprising at least one compound having thestructure IV.

Furthermore, it is possible to use a primary antioxidant comprisingexclusively compounds selected from among

-   -   tri-tert-butylphenols, in particular 2,4,6-tri-tert-butylphenol,    -   di-tert-butylphenols, in particular 2,4-di-tert-butylphenol,        2,5-di-tert-butylphenol, 2,6-di-tert-butylphenol,    -   di-tert-butylmethylphenols, in particular        2,5-di-tert-butyl-4-methylphenol,        2,6-di-tert-butyl-4-methylphenol,        4,6-di-tert-butyl-2-methylphenol, di-tert-butyl-3-methylphenol,    -   tert-butylmethylphenols, in particular        2-tert-butyl-4-methylphenol, 6-tert-butyl-2-methylphenol,        4-tert-butyl-2-methylphenol, 6-tert-butyl-3-methylphenol,    -   tert-butyldimethylphenols, in particular        4-tert-butyl-2,6-dimethylphenol,        6-tert-butyl-2,4-dimethylphenol, tert-butyl-2,5-dimethylphenol,    -   tert-butylphenols, in particular 2-tert-butylphenol,        4-tert-butylphenol,    -   di-sec-butylphenols,    -   sec-butylphenols, in particular 2-sec-butylphenol,        2-sec-butyl-4-tert-butylphenol,        4-sec-butyl-2,6-di-tert-butylphenol,    -   tert-amylphenols, in particular 2-tert-amylphenol,    -   di-tert-amylphenols, in particular 2,4-di-tert-amylphenol,    -   2-isopropylphenol, 4-octylphenol, 4-nonylphenol,        2,6-di-tert-butyl-4-nonylphenol, 4-dodecylphenol and/or        octadecylphenol,        and mixtures of these compounds, with this primary antioxidant        having a melting point of less than or equal to 40° C.

Pure substances or mixtures which are or comprise compounds having thestructures I to IV and have a melting point of greater than 40° C. areexcluded from the present invention.

The oxidation-stabilized biodiesel of the invention comprises from 10 to20 000 ppm (w/w), preferably from 50 to 12 000 ppm (w/w) and morepreferably from 100 to 8000 ppm (w/w), of a primary antioxidant whichhas a melting point of less than or equal to 40° C. and comprises atleast one compound having the structure I.

The primary antioxidant of the oxidation-stabilized biodiesel of theinvention therefore preferably comprises exclusively alkyphenols and/orsubstituted alkylphenols. In particular, the oxidation-stabilizedbiodiesel of the invention comprises a mixture of various substitutedand/or unsubstituted alkylphenols as primary antioxidant. The biodieselof the present invention preferably comprises a primary antioxidantcomprising from 0.1 to 99.9% by weight, more preferably from 50 to 99%by weight and particularly preferably from 55 to 99% by weight, of oneor more compounds having the structure I. The biodiesel of the inventionparticularly preferably comprises a mixture comprising exclusivelycompounds having the structure I as primary antioxidant.

The biodiesel of the invention preferably comprises a primaryantioxidant comprising at least one compound having the structure II.The biodiesel of the invention more preferably comprises a primaryantioxidant comprising at least one compound having the structure III.The biodiesel of the invention particularly preferably comprises aprimary antioxidant comprising at least one compound having thestructure IV.

Furthermore, the biodiesel of the invention can comprise a primaryantioxidant comprising exclusively compounds selected from among

-   -   tri-tert-butylphenols, in particular 2,4,6-tri-tert-butylphenol,    -   di-tert-butylphenols, in particular 2,4-di-tert-butylphenol,        2,5-di-tert-butylphenol, 2,6-di-tert-butylphenol,    -   di-tert-butylmethylphenols, in particular        2,5-di-tert-butyl-4-methylphenol,        2,6-di-tert-butyl-4-methylphenol,        4,6-di-tert-butyl-2-methylphenol, di-tert-butyl-3-methylphenol,    -   tert-butylmethylphenols, in particular        2-tert-butyl-4-methylphenol, 6-tert-butyl-2-methylphenol,        4-tert-butyl-2-methylphenol, 6-tert-butyl-3-methylphenol,    -   tert-butyldimethylphenols, in particular        4-tert-butyl-2,6-dimethylphenol, 6-tert-butyl        -2,4-dimethylphenol, tert-butyl-2,5-dimethylphenol,    -   tert-butylphenols, in particular 2-tert-butylphenol,        4-tert-butylphenol,    -   di-sec-butylphenols,    -   sec-butylphenols, in particular 2-sec-butylphenol,        2-sec-butyl-4-tert-butylphenol,        4-sec-butyl-2,6-di-tert-butylphenol,    -   tert-amylphenols, in particular 2-tert-amylphenol,    -   di-tert-amylphenols, in particular 2,4-di-tert-amylphenol,    -   2-isopropylphenol, 4-octylphenol, 4-nonylphenol,        2,6-di-tert-butyl-4-nonylphenol, 4-dodecylphenol and/or        octadecylphenol,        and mixtures of these compounds.

In particular, the biodiesel of the invention comprises C₁₂-C₂₄ fattyacid alkyl esters, preferably C₁₂-C₂₄ fatty acid methyl esters orC₁₂-C₂₄ fatty acid ethyl esters, which can be present in pure form or asa mixture. In addition, the biodiesel of the invention can furthercomprise all customary additives such as secondary antioxidants,antifoams. The biodiesel of the invention preferably comprisestransesterification products of rapeseed oil, soybean oil, sunfloweroil, palm kernel oil, coconut oil, jatropha oil and/or used cookingoils. The biodiesel of the invention particularly preferably comprisestransesterification products obtained from rapeseed oil, sunflower oilor soybean oil by transesterification. The biodiesel of the inventioncan also comprise mixtures of transesterification products of variousvegetable and/or animal oils.

In addition, the biodiesel of the invention can further comprise allcustomary additives such as secondary antioxidants, antifoams,low-temperature flow improvers. Secondary antioxidants which can bepresent in the biodiesel of the invention are alkylthio-methylphenols,preferably selected from among

-   -   2,4-di((octylthio)methyl)-6-tert-butylphenol,    -   2,4-di((octylthio)methyl)-6-methylphenol,    -   2,4-di((octylthio)methyl)-6-ethylphenol and    -   2,6-di((dodecylthio)methyl)-4-nonylphenol,        hydroxylated diphenyl thioethers, preferably selected from among    -   2,2′-thiobis[6-tert-butyl-4-methylphenol],    -   2,2′-thiobis[4-octylphenol],    -   4,4′-thiobis[6-tert-butyl-3-methylphenol],    -   4,4′-thiobis[6-tert-butyl-2-methylphenol],    -   4,4′-thiobis[3,6-di-sec-amylphenol] and    -   4,4′-bis[2,6-dimethyl-4-hydroxyphenyl]disulfide,        phosphites or phosphonites, preferably selected from among    -   triphenyl phosphite,    -   diphenyl alkyl phosphites,    -   phenyl dialkyl phosphites,    -   tris[nonylphenyl] phosphite,    -   trilauryl phosphite,    -   trioctadecyl phosphite,    -   distearyl pentaerythrityl diphosphite,    -   tris[2,4-di-tert-butylphenyl] phosphite,    -   diisodecyl pentaerythrityl diphosphite,    -   bis[2,4-di-tert-butylphenyl] pentaerythrityl diphosphite,    -   bis[2,6-di-tert-butyl-4-methylphenyl] pentaerythrityl        diphosphite,    -   bis[isodecyloxy] pentaerythrityl diphosphite,    -   bis[2,4-di-tert-butyl-6-methylphenyl] pentaerythrityl        diphosphite,    -   bis[2,4,6-tri-tert-butylphenyl] pentaerythrityl diphosphite,    -   tristearyl sorbitol triphosphite,    -   tetrakis[2,4-di-tert-butylphenyl] 4,4′-biphenylenediphosphonite,    -   6-isooctyloxy-2,4,8,10-tetra-tert-butyl-12H-dibenzo[d,g]-1,3,2-dioxaphosphocine,    -   6-fluoro-2,4,8,10-tetra-tert-butyl-12-methyldibenzo[d,g]-1,3,2-dioxaphosphocine,    -   bis[2,4-di-tert-butyl-6-methylphenyl] methyl phosphite and    -   bis[2,4-di-tert-butyl-6-methylphenyl]ethyl phosphite,        or peroxide-destroying compounds, preferably selected from among    -   esters of β-thiodipropionic acid, preferably the lauryl,        stearyl, myristyl or tridecyl ester,    -   mercaptobenzimidazole,    -   the zinc salt of 2-mercaptobenzimidazole,    -   zinc dibutyldithiocarbamate,    -   dioctadecyl disulfide and    -   pentaerythrityl tetrakis[β-dodecylmercapto]propionate,        or mixtures of these compounds.

These secondary antioxidants can be present in an amount of from 10 to20 000 ppm (w/w), preferably from 50 to 12 000 ppm (w/w) and morepreferably from 100 to 8000 ppm (w/w), in the biodiesel of theinvention.

The biodiesel of the invention is preferably produced using the methodof the invention.

EXAMPLES

The following examples illustrate the method of the invention withoutrestricting the invention to this embodiment.

Example 1 Production of the Samples

In a glass beaker, the primary antioxidant is dissolved in biodiesel at20° C. with stirring, and stirring is continued until a clear solutionof the primary antioxidant in the biodiesel is obtained. The primaryantioxidants used, the biodiesel used and the ratios are shown in Table1.

Example 2 Composition of the Primary Antioxidants Used

Primary Antioxidant 1

(procured from Degussa AG under the trade name IONOL 99): >99.0% byweight  of 2,6-di-tert-butylphenol <0.5% by weight of 2-tert-butylphenol<0.5% by weight of 2,4-di-tert-butylphenolPrimary Antioxidant 2

(procured from Degussa AG under the trade name IONOL K98): >98.5% byweight of 6-tert-butyl-2,4-dimethylphenol  <1.5% by weight of4-tert-butyl-2,6-dimethylphenol and di-tert-butylmethyl-phenolsPrimary Antioxidant 3

(procured from Degussa AG under the trade name IONOL K65): >55% byweight of 6-tert-butyl-2,4-dimethylphenol >15% by weight of2,6-di-tert-butyl-4-methylphenol 18-22% by weight oftert-butyl-2,5-dimethylphenol <1% by weight of4,6-di-tert-butyl-2-methylphenol <3.5% by weight ofdi-tert-butyl-3-methylphenol <3.5% by weight of2,5-di-tert-butyl-4-methylphenolPrimary Antioxidant 4

(procured from Degussa AG under the trade name IONOL 75): >75% by weightof 2,6-di-tert-butylphenol <5% by weight of 2-tert-butylphenol <0.5% byweight of 4-tert-butylphenol <3% by weight of 2,4-di-tert-butylphenol<1% by weight of 2,5-di-tert-butylphenol <15.5% by weight of2,4,6-tri-tert-butylphenol

Example 3 Testing Procedure

The oxidation stability of the samples produced as described in Example1 was examined at a test temperature of 110° C. in accordance with thetest method DIN EN 14112.

Example 4 Results of the Test Method

Amount of liquid antioxidant Oxidation stability Antioxidant [in ppm][in h at 110° C.] Rapeseed oil methyl ester — — 5.1 1  500 6.7 2  5006.5 3  500 6.6 4  500 6.7 Baynox* 2500 7.1 Used cooking fat methyl ester— — 4.0 1 2000 9.7 2 2000 9.5 3 2000 9.1 4 2000 9.4 Baynox* 10 000  12.0  Soybean oil methyl ester — — 3.6 1 2000 7.3 2 2000 6.6 3 2000 6.34 2000 7.1 Baynox* 10 000   8.8 Sunflower oil methyl ester — — 1.6 14000 8.8 2 4000 7.8 3 4000 6.9 4 4000 8.0 BAYNOX* 20 000   9.0*BAYNOX is an antioxidant from Lanxess and is a solution of thedistilled, highly pure active ingredient in biodiesel. The activecontent has been set to 20% g/l, so that 1 l of BAYNOX corresponds to200 g of active ingredient. Analysis showed that the active ingredientin BAYNOX is 2,6-di-tert-butyl-4-hydroxytoluene.

This application is based on German patent application No.102005015474.3, filed on Apr. 4, 2005, and incorporated herein byreference.

1. A method of increasing the oxidation stability of biodiesel,comprising: adding a primary antioxidant having a melting point of lessthan or equal to 40° C. to a biodiesel in an amount of from 10 to 20 000ppm (w/w), wherein the primary antioxidant comprises at least onecompound having the structure

wherein R₁, R₂=hydrogen, a linear alkyl group having from 1 to 20 carbonatoms or

wherein * is a carbon atom of the aromatic ring system, R₃, R₅=hydrogen,a linear alkyl group having from 1 to 20 carbon atoms, R₄=hydrogen, alinear alkyl group having from 1 to 40 carbon atoms, wherein both R₁ andR₂ and R₃ and R₅ are in each case identical or different.
 2. The methodas claimed in claim 1, wherein the primary antioxidant comprising from0.1 to 99.9% by weight of one or more compounds having the structure Iis used as primary antioxidant.
 3. The method as claimed in claim 1,wherein a mixture comprising exclusively compounds having the structureI is used as the primary antioxidant.
 4. The method as claimed in claim1, wherein the primary antioxidant comprises at least one compoundhaving the structure


5. The method as claimed in claim 1, wherein the primary antioxidantcomprises at least one compound having the structure


6. The method as claimed in claim 1, wherein the compound represented bystructure I is added in the form of a liquid.
 7. The method as claimedin claim 1, wherein the primary antioxidant comprises at least onemember selected from the group consisting of tri-tert-butylphenols,di-tert-butylphenols, di-tert-butylmethylphenols,tert-butylmethylphenols, tert-butyldimethylphenols, tert-butylphenols,di-sec-butylphenols, sec-butylphenols, tert-amylphenols anddi-tert-amylphenols.
 8. The method as claimed in claim 1, wherein theprimary antioxidant comprises at least one member selected from thegroup consisting of 2,4,6-tri-tert-butylphenol, 2,4-di-tert-butylphenol,2,5-di-tert-butylphenol, 2,6-di-tert-butylphenol,2,5-di-tert-butyl-4-methylphenol, 2,6-di-tert-butyl-4-methylphenol,4,6-di-tert-butyl-2-methylphenol, di-tert-butyl-3-methylphenol,2-tert-butyl-4-methylphenol, 6-tert-butyl -2-methylphenol,4-tert-butyl-2-methylphenol, 6-tert-butyl-3-methylphenol, 4-tert-butyl-2,6-dimethylphenol, 6-tert-butyl-2,4-dimethylphenol,tert-butyl-2,5-dimethylphenol, 2-tert-butylphenol, 4-tert-butylphenol,2-sec-butylphenol, 2-sec -butyl-4-tert-butylphenol,4-sec-butyl-2,6-di-tert-butylphenol, 2-tert-amylphenol,2,4-di-tert-amylphenol, 2-isopropylphenol, 4-octylphenol, 4-nonylphenol,2,6-di-tert -butyl-4-nonylphenol, 4-dodecylphenol and octadecylphenol.9. The method as claimed in claim 1, in which a secondary antioxidant isadded to the biodiesel.
 10. The method as claimed in claim 9, whereinthe secondary antioxidant comprises at least one member selected fromthe group consisting of alkylthiomethylphenols, hydroxylated diphenylthioethers, phosphates and phosphonites and peroxide-destroyingcompounds
 11. The method as claimed in claim 9, wherein the secondaryantioxidant comprises at least one member selected from the groupconsisting of 2,4-di((octylthio)methyl)-6-tert-butylphenol,2,4-di((octylthio)methyl)-6-methylphenol,2,4-di((octylthio)methyl)-6-ethylphenol,2,6-di((dodecylthio)methyl)-4-nonylphenol,2,2′-thiobis[6-tert-butyl-4-methylphenol], 2,2′-thiobis[4-octylphenol],4,4′-thiobis[6-tert-butyl-3-methylphenol],4,4′-thiobis[6-tert-butyl-2-methylphenol],4,4′-thiobis[3,6-di-sec-amylphenol],4,4′-bis[2,6-dimethyl-4-hydroxyphenyl]disulfide, triphenyl phosphite,diphenyl alkyl phosphites, phenyl dialkyl phosphites, tris[nonylphenyl]phosphite, trilauryl phosphite, trioctadecyl phosphite, distearylpentaerythrityl diphosphite, tris[2,4-di-tert-butylphenyl] phosphite,diisodecyl pentaerythrityl diphosphite, bis[2,4-di-tert-butylphenyl]pentaerythrityl diphosphite, bis[2,6-di-tert-butyl-4-methylphenyl]pentaerythrityl diphosphite, bis[isodecyloxy] pentaerythrityldiphosphite, bis[2,4-di-tert-butyl-6-methylphenyl] pentaerythrityldiphosphite, bis[2,4,6-tri-tert-butylphenyl] pentaerythrityldiphosphite, tristearyl sorbitol triphosphite,tetrakis[2,4-di-tert-butylphenyl] 4,4′-biphenylenediphosphonite,6-isooctyloxy-2,4,8,10-tetra-tert-butyl-12H-dibenzo[d,g]-1,3,2-dioxaphosphocine,6-fluoro-2,4,8,10-tetra-tert-butyl-12-methyldibenzo[d,g]-1,3,2-dioxaphosphocine,bis[2,4-di-tert-butyl-6-methylphenyl] methyl phosphate,bis[2,4-di-tert-butyl-6-methylphenyl] ethyl phosphite, esters ofβ-thiodipropionic acid, mercaptobenzimidazole, the zinc salt of2-mercaptobenzimidazole, zinc dibutyldithiocarbamate, dioctadecyldisulfide and pentaerythrityl tetrakis[β-dodecylmercapto]propionate. 12.The method as claimed in claim 1, wherein the biodiesel comprises asaturated and/or unsaturated alkyl ester of a fatty acid.
 13. The methodas claimed in claim 1, wherein the biodiesel comprises a mixture of asaturated and/or unsaturated fatty acid alkyl ester with a liquid energycarrier.
 14. The method as claimed in claim 1, wherein the primaryantioxidant is added in an amount of from 50 to 12 000 ppm (w/w). 15.The method as claimed in claim 1, wherein the primary antioxidant isadded in an amount of from 100 to 8000 ppm (w/w).
 16. The method asclaimed in claim 1, wherein the primary antioxidant is added to thebiodiesel at a temperature of from 18° C. to 60° C.
 17. The method asclaimed in claim 1, wherein the primary antioxidant is added to thebiodiesel at a temperature of from 20° C. to 40° C.
 18. Anoxidation-stabilized biodiesel, comprising: from 10 to 20 000 ppm (w/w)of a primary antioxidant which has a melting point of less than or equalto 40° C. and comprises at least one compound having the structure

wherein R₁, R₂=hydrogen, a linear alkyl group having from 1 to 20 carbonatoms or

wherein * is a carbon atom of the aromatic ring system, R₃, R₅=hydrogen,a linear alkyl group having from 1 to 20 carbon atoms, R₄=hydrogen, alinear alkyl group having from 1 to 40 carbon atoms, wherein both R₁ andR₂ and R₃ and R₅ are in each case identical or different.
 19. The methodas claimed in claim 1, wherein the primary antioxidant comprising from0.1 to 99.9% by weight of one or more compounds having the structure Iis used as primary antioxidant.
 20. The biodiesel as claimed in claim18, wherein a mixture comprising exclusively compounds having thestructure I is used as the primary antioxidant.
 21. The biodiesel asclaimed in claim 18, wherein the primary antioxidant comprises at leastone compound having the structure


22. The biodiesel as claimed in claim 18, wherein the primaryantioxidant comprises at least one compound having the structure


23. The biodiesel as claimed in claim 18, wherein the primaryantioxidant comprises at least one member selected from the groupconsisting of tri-tert -butylphenols, di-tert-butylphenols,di-tert-butylmethylphenols, tert -butylmethylphenols,tert-butyldimethylphenols, tert-butylphenols, di-sec -butylphenols,sec-butylphenols, tert-amylphenols and di-tert-amylphenols.
 24. Thebiodiesel as claimed in claim 18, wherein the primary antioxidantcomprises at least one member selected from the group consisting of2,4,6-tri-tert -butylphenol, 2,4-di-tert-butylphenol,2,5-di-tert-butylphenol, 2,6-di-tert-butylphenol,2,5-di-tert-butyl-4-methylphenol, 2,6-di-tert-butyl-4-methylphenol,4,6-di-tert-butyl -2-methylphenol, di-tert-butyl-3-methylphenol,2-tert-butyl-4-methylphenol, 6-tert -butyl-2-methylphenol,4-tert-butyl-2-methylphenol, 6-tert-butyl-3-methylphenol,4-tert-butyl-2,6-dimethylphenol, 6-tert-butyl-2,4-dimethylphenol,tert-butyl-2,5-dimethylphenol, 2-tert-butylphenol, 4-tert-butylphenol,2-sec-butylphenol, 2-sec -butyl-4-tert-butylphenol,4-sec-butyl-2,6-di-tert-butylphenol, 2-tert-amylphenol,2,4-di-tert-amylphenol, 2-isopropylphenol, 4-octylphenol, 4-nonylphenol,2,6-di-tert -butyl-4-nonylphenol and 4-dodecylphenol andoctadecylphenol.
 25. The biodiesel as claimed in claim 18, furthercomprising a secondary antioxidant.
 26. The biodiesel as claimed inclaim 25, wherein the secondary antioxidant comprises at least onemember selected from the group consisting of alkylthiomethylphenols,hydroxylated diphenyl thioethers, phosphates, phosphonites andperoxide-destroying compounds
 27. The biodiesel as claimed in claim 25,wherein the secondary antioxidant comprises at least one member selectedfrom the group consisting of2,4-di((octylthio)methyl)-6-tert-butylphenol,2,4-di((octylthio)methyl)-6-methylphenol,2,4-di((octylthio)methyl)-6-ethylphenol,2,6-di((dodecylthio)methyl)-4-nonylphenol,2,2′-thiobis[6-tert-butyl-4-methylphenol], 2,2′-thiobis[4-octylphenol],4,4′-thiobis[6-tert-butyl-3-methylphenol],4,4′-thiobis[6-tert-butyl-2-methylphenol],4,4′-thiobis[3,6-di-sec-amylphenol],4,4′-bis[2,6-dimethyl-4-hydroxyphenyl]disulfide, triphenyl phosphite,diphenyl alkyl phosphites, phenyl dialkyl phosphites, tris[nonylphenyl]phosphite, trilauryl phosphite, trioctadecyl phosphite, distearylpentaerythrityl diphosphite, tris[2,4-di-tert-butylphenyl] phosphite,diisodecyl pentaerythrityl diphosphite, bis[2,4-di-tert-butylphenyl]pentaerythrityl diphosphite, bis[2,6-di-tert-butyl-4-methylphenyl]pentaerythrityl diphosphite, bis[isodecyloxy] pentaerythrityldiphosphite, bis[2,4-di-tert-butyl-6-methylphenyl] pentaerythrityldiphosphite, bis[2,4,6-tri-tert-butylphenyl] pentaerythrityldiphosphite, tristearyl sorbitol triphosphite,tetrakis[2,4-di-tert-butylphenyl] 4,4′-biphenylenediphosphonite,6-isooctyloxy-2,4,8,10-tetra-tert-butyl-12H-dibenzo[d,g]-1,3,2-dioxaphosphocine,6-fluoro-2,4,8,10-tetra-tert-butyl-12-methyldibenzo[d,g]-1,3,2-dioxaphosphocine,bis[2,4-di-tert-butyl-6-methylphenyl] methyl phosphate,bis[2,4-di-tert-butyl-6-methylphenyl] ethyl phosphite, esters ofβ-thiodipropionic acid, mercaptobenzimidazole, the zinc salt of2-mercaptobenzimidazole, zinc dibutyldithiocarbamate, dioctadecyldisulfide and pentaerythrityl tetrakis[β-dodecylmercapto]propionate. 28.The biodiesel as claimed in claim 18, which comprises a saturated and/orunsaturated alkyl ester of a fatty acid.
 29. The biodiesel as claimed inclaim 18, which comprises a saturated and/or unsaturated fatty acidalkyl ester and a liquid energy carrier.
 30. The biodiesel as claimed inclaim 18, which comprises the primary antioxidant in an amount of from50 to 12 000 ppm (w/w).
 31. The biodiesel as claimed in claim 18, whichcomprises the primary antioxidant in an amount of from 100 to 8000 ppm(w/w).